1. Field
The present disclosure relates generally to sensors used to indicate if a packaged material has been previously heated to above certain temperature level, and in particular to sensors that can be placed in or on packaged food that are supposed to be kept below certain temperature, such as below freezing point, to indicate if the packaged food has been previously heated above a certain temperature level, for example, whether it has been wholly or partially defrosted in storage or during transportation.
2. Prior Art
In the food industry in particular, many foods are preserved in frozen form to keep them fresh. In such cases, foods of various sorts are frozen soon after appropriate preparations, and are supposed to be kept frozen until shortly prior to use. In general, the frozen material is required to be kept frozen to prevent spoilage. In certain cases, for example for the case of fish, the fish is frozen soon after catching (with or without any processing) and are to be kept frozen until obtained by a user, such as a food processor or a final user (consumer), otherwise its value is significantly reduced and in certain cases may even have to be discarded.
In many cases, the frozen food is transported over long distances and stays stored for some length of time before it reaches the final user. At that point, a processor or the final user is eager to know if the frozen food has ever been defrosted since its initial freezing. The frozen food is usually packaged in sealed plastic bags for low cost and to prevent contamination and are marked and sometimes dated so that the user can become aware of the content without requiring opening the packaging.
It is therefore highly desirable to determine if the packaged and sealed frozen food has ever been defrosted since its initial freezing. Hereinafter, by the term “defrosting” it is intended to indicate the rise in temperature above some predetermined temperature that may cause an undesirable state for the food, such as a decrease in quality or spoilage. For example, defrosting would indicate the temperature rising above 32 degrees F. (0 degree C.), i.e., the melting point of ice. However, for frozen food, it may also indicate a temperature rise above a point at which the frozen food is desired to be kept, e.g., 10 degrees F. (around −12 degrees C.). Hereinafter, such devices will be generally referred to as “defrost indicator” even though they may indicate the food being subjected to undesirable temperatures other than that at which the frozen food begins to defrost.
As discussed above, the aforementioned “freezing temperature” may also be used to indicate almost any temperature below which the packaged food or other material is desired to be kept. For example, certain fish, eggs or vegetables may be desired to be kept below 5 or 10 degrees C. The present “defrost indicators” are therefore meant to indicate any desired temperature below which the packaged material is desired to be kept and not solely the actual freezing temperature of the packaged material.
In the following descriptions of the various embodiments, the “defrost indicator” is considered to be placed inside (or on) a packaged frozen food (e.g., under a transparent portion of the packaging—for example clear plastic sheeting) or its container so that the user can observe the state of the packaged food, e.g., whether it has been defrosted after initial freezing. It is, however, appreciated by those skilled in the art that such “defrost indicators” may also be placed within the food and otherwise not observable from the outside of the packaging. In such cases, the final user has to open the package to expose the defrost indicator to determine if the food has been defrosted after its initial freezing. In addition, such defrost indicators may also be placed in bulk storage bins or the like for the same defrost indication purposes. In all such applications, those embodiments that allow certain amount of higher temperature exposure before indicating past defrosting are preferably used so that false defrost indication is avoided.
Numerous devices exist for measuring the temperature in general and the temperature of packaged material of different types. Numerous mercury type and battery operated thermometers (the so-called “high/low thermometers”) are also available that can display or record the highest and the lowest temperatures that they have been exposed to over any length of time. Such high/low thermometers are, however, not practical for packaged frozen food due to their cost, size, survival during handling, and for the case of battery operated devices the existence of a battery itself as a hazardous material and the fact that the battery has to power the device over very long periods of time and in extreme environments.